Three dimensional stacked nonvolatile semiconductor memory

ABSTRACT

In a three dimensional stacked nonvolatile semiconductor memory according to the present invention, a first block has a selected first cell unit including a memory cell to be read and a non-selected second cell unit not including a memory cell to be read. A read potential or a transfer potential higher than the read potential is applied to the word line in the first block in a state that a ground potential is applied to a channel of a memory cell existing nearer to the bit line side than a memory cell in the second cell unit to which the read potential is applied, after which all the memory cells in the second cell unit are cut off from the bit line, the bit line is set to a precharge potential, and read is performed to the a memory cell to be read in the first cell unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 13/341,123 filed Dec. 30, 2011,which is a continuation of Ser. No. 12/953,193 filed Nov. 23, 2010 (nowU.S. Pat. No. 8,107,292 issued Jan. 31, 2012), which is a continuationof U.S. Ser. No. 12/407,494 filed Mar. 19, 2009 (now U.S. Pat. No.7,852,676 issued Dec. 14, 2010), and claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2008-112660filed Apr. 23, 2008, the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a three dimensional stacked nonvolatilesemiconductor memory.

2. Description of the Related Art

BiCS (Bit Cost Scalable) technology is known as a technology forsuppressing a bit cost of a semiconductor memory by increasing thecapacity thereof by a three dimensional structure (refer to, forexample, “Bit Cost Scalable Technology with Punch and Plug Process forUltra High Density Flash Memory” 2007 Symposium on VLSI TechnologyDigest of Technical Papers. p. 14).

A nonvolatile semiconductor memory to which the BiCS technology isapplied (hereinafter, called a BiCS memory) has a feature in that it notonly has a three dimensional structure but makes bit cost scalabilitypossible so that a bit cost can be reduced in proportion to an increaseof the number of stacked layers by devising a device structure and aprocess technology.

In, for example, a NAND flash memory to which the BiCS technology isapplied (hereinafter, called a BiCS-NAND flash memory), a memorycapacity, which greatly exceeds the limit of the memory capacity of aNAND flash memory having a two-dimensional structure, can be realized byincreasing the number of cells in a longitudinal direction whichcomprise a NAND column by increasing the number of stacked layers.

However, since the BiCS memory which is represented by a BiCS-NAND flashmemory has a unique device structure, there are many problems to besolved to practically use the BiCS memory.

A read disturb is exemplified as one of the problems.

The BiCS memory has such a feature that cell units are included in oneblock connected to one bit line. Further, the cell units cannot beselected at the same time from the viewpoint of a circuit operation.Accordingly, a non-selected cell unit which does not include a memorycell to be read exists in a selected block.

This problem does not occur in a flash memory having a two-dimensionalstructure.

Therefore, read disturb must be examined to prevent variation of athreshold voltage of a memory cell in a non-selected cell unit in aselected block in read.

In particular, in the BiCS memory, since it is not necessary to apply aread potential and a transfer potential to the cell unit in thenon-selected block unlike the flash memory having the two-dimensionalstructure, it is not necessary to examine read disturb to the cell unitin the non-selected block. However, when a read potential is applied toa non-selected memory cell because a non-selected cell unit in aselected block is cut off from a bit line, a threshold voltage is variedin a non-selected memory cell existing nearer to the bit line side thanthe above non-selected memory cell by read disturb depending on thestate of a threshold voltage of the non-selected memory cell to whichthe read potential is applied.

BRIEF SUMMARY OF THE INVENTION

A three dimensional stacked nonvolatile semiconductor memory accordingto an aspect of the present invention comprises a semiconductorsubstrate, a memory cell array comprised of first and second blocksdisposed on the semiconductor substrate side by side in a firstdirection, and a first driver disposed on one end of the memory cellarray in a second direction orthogonal to the first direction.

Each of the first and second blocks is comprised of at least threeconductive layers stacked on the semiconductor substrate by beinginsulated from each other, a bit line disposed on the at least threeconductive layers by being insulated therefrom, and columnarsemiconductors having lower ends connected to the semiconductorsubstrate and upper ends connected to the bit line and passing throughthe at least three conductive layers.

The uppermost layer of the at least three conductive layers is comprisedof first select gate lines extending in the second direction, thelowermost layer of the at least three conductive layers is a secondselect gate line, the remaining conductive layers excluding theuppermost layer and the lowermost layer of the at least three conductivelayers are a word line, and the remaining conductive layers excludingthe uppermost layer of the at least three conductive layers have a plateshape whose width in the first direction is larger than the width in thefirst direction of the first select gate lines.

Select gate transistors are comprised of the first select gate lines andthe columnar semiconductors, and the second select gate line and thecolumnar semiconductors, respectively and memory cells are comprised ofthe word line and the columnar semiconductors, respectively.

The first block has a selected first cell unit including a memory cellto be read and a non-selected second cell unit not including a memorycell to be read.

A read potential or a transfer potential higher than the read potentialis applied to the word line in the first block in a state that a groundpotential is applied to a channel of a memory cell existing nearer tothe bit line side than a memory cell in the second cell unit to whichthe read potential is applied, after which all the memory cells in thesecond cell unit are cut off from the bit line, the bit line is set to aprecharge potential, and read is performed to the memory cell to be readin the first cell unit.

Further, the read potential and the transfer potential are not appliedto the word line in the second block in the read.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a bird's eye view of a BiCS-NAND flash memory;

FIG. 2 is a plan view of the BiCS-NAND flash memory;

FIG. 3 is a sectional view taken along line III-III of FIG. 2;

FIG. 4 is a sectional view taken along line IV-IV of FIG. 2;

FIG. 5 is a structure view of a NAND cell unit;

FIG. 6 is a bird's eye view of the NAND cell unit;

FIG. 7 is an equivalent circuit view of a memory cell array;

FIG. 8 is a comparative view comparing a BiCS-NAND with atwo-dimensional NAND;

FIG. 9 is a view showing a first example of a block layout;

FIG. 10 is a view showing a second example of the block layout;

FIG. 11 is a view showing a third example of the block layout;

FIG. 12 is a view showing a fourth example of the block layout;

FIG. 13 is a view showing a layout of select gate lines on a bit lineside;

FIG. 14 is a plan view when the select gate lines shown in FIG. 13 arearranged as a device;

FIG. 15 is a view showing a layout of select gate lines on a bit lineside;

FIG. 16 is a plan view when the select gate lines shown in FIG. 15 arearranged as a device;

FIG. 17 is a view showing a layout of select gate lines on a bit lineside;

FIG. 18 is a view showing an example of a driver circuit;

FIG. 19 is a view explaining read disturb of a BiCS memory;

FIG. 20 is a view explaining read disturb of the BiCS memory;

FIG. 21 is a timing view showing a read method as a reference example;

FIG. 22 is a view showing the relation of potentials of the referenceexample;

FIG. 23 is a view showing the relation of potentials of the referenceexample;

FIG. 24 is a view showing how read disturb occurs;

FIG. 25 is a timing view showing a read method according to an exampleof the present invention;

FIG. 26 is a view showing the relation of potentials according to anexample of the present invention;

FIG. 27 is a view showing the relation of potentials according to anexample of the present invention; and

FIG. 28 is a view showing how read disturb is improved.

DETAILED DESCRIPTION OF THE INVENTION

A three dimensional stacked nonvolatile semiconductor memory of anaspect of the present invention will be described below in detail withreference to the accompanying drawing.

1. Outline

An example of the present invention proposes a read method of preventingread disturb, which is caused by a structure specific to a BiCS memory,i.e., variation of a threshold value of a memory cell in a non-selectedcell unit in a selected block.

When a feature of the BiCS memory is expressed simply from the viewpointof a circuit, cell units are included in one block connected to one bitline. In this case, the cell units cannot be selected at the same timefrom the viewpoint of a circuit operation. Accordingly, in read, one ofcell units connected to the one bit line must be selected.

That is, a non-selected cell unit, which does not include a memory cellto be read in the selected block, is cut off from the bit line in theread.

Further, when a non-selected memory cell, to which a read potential isapplied, in the non-selected cell unit, is in a high threshold valuestate, i.e., for example, a “0” state of a binary value (“0”/“1”) storedtherein, the non-selected memory cell keeps a turned-off state beforeand after the read potential is applied thereto.

Accordingly, a channel of a memory cell existing nearer to the bit lineside than the memory cell, to which the read potential is applied, inthe non-selected cell unit, is in a floating state as well as is boostedby capacitance coupling when the read potential is applied to the memorycell.

In contrast, in the BiCS memory, a ground potential is transferred froma source line to a channel of a memory cell existing nearer to thesource line side than the memory cell, to which the read potential isapplied, in the non-selected cell unit, in read.

Accordingly, since a large potential difference is generated between asource and a drain of the memory cell, to which the read potential isapplied, in the non-selected cell unit, punch-through is caused betweenthe source and the drain of the memory cell.

A hot carrier is generated by the punch-through and injected into acharge accumulation layer of a memory cell existing nearer to the bitline side than the memory cell, to which the read potential is applied,in the non-selected cell unit, and a threshold voltage is variedthereby.

To prevent the read disturb, it is preferable to set operation timing atwhich generation of a large potential difference is prevented betweenthe source and the drain of the memory cell to which the read potentialis applied, in the non-selected cell unit.

Thus, the example of the present invention proposes a read method ofapplying a ground potential to a channel of a memory cell existingnearer to a bit line side than a memory cell, to which a read potentialis applied, in a non-selected cell unit in a block selected in read tothereby prevent channel boost.

Specifically, the read potential or a transfer potential higher than theread potential is applied to a word line in the selected block in astate that the ground potential is applied to the channel of the memorycell existing nearer to the bit line side than the memory cell, to whichthe read potential is applied, in the non-selected cell unit in theselected block, after which all the memory cells in the non-selectedcell unit are cut off from the bit line, the bit line is set to aprecharge potential, and read is performed to a memory cell to be readin the selected cell unit.

The ground potential is transferred from, for example, the bit line tothe channel of memory cell existing nearer to the bit line side than thememory cell, to which the read potential is applied, in the non-selectedcell unit.

With this operation, when the read potential or the transfer potentialhigher than the read potential is applied to the word line in theselected block, the channel of the memory cell existing nearer to thebit line side than the memory cell, to which the read potential isapplied, in the non-selected cell unit is not boosted, and thus readdisturb can be improved.

2. BiCS memory

(1) Basic Structure

First, a basic structure of a BiCS memory will be explained.

FIG. 1 shows a bird's eye view of a BiCS-NAND flash memory.

The NAND flash memory is comprised of blocks each of which acts, forexample, as a unit to be erased. Here, two blocks BK<i>, BK<i+1> areshown in the figure.

A source diffusion layer 24 formed in a semiconductor substrate isshared by, for example, all the blocks. The source diffusion layer 24 isconnected to a source line SL·M1 through a contact plug P_(SL). Further,at least three conductive layers (in the example, six-layer structure)comprised of, for example, conductive polysilicon are laminated on thesource diffusion layer 24.

The remaining five conductive layers excluding the uppermost layer areformed in a plate shape, respectively in the one block BK<i+1> as wellas the ends thereof in the X-direction are formed stepwise so that theyare in contact with the respective conductive layers. A lowermost layeracts as a select gate line SGS on the source line side, and theremaining four conductive layers excluding the lowermost and uppermostlayers act as word lines WL<0>, WL<1>, WL<2>, and WL<3>.

The uppermost layer is comprised of line-shaped conductive wiresextending in the X-direction. Six conductive wires, for example, aredisposed in the one block BK<i+1>. The six conductive wires, forexample, of the uppermost layer act as select gate lines SGD<0>, . . . ,SGD<5> on a bit line side.

Active layers (active areas) AA for constituting a NAND cell unit areformed columnarly in a Z-direction (direction vertical to the frontsurface of the semiconductor substrate) so that they reach the sourcediffusion layer 24 passing through the conductive layers.

The upper ends of the active layers AA are connected to bit lines BL<0>,. . . , BL<m> extending in a Y-direction. Further, the select gate lineSGS on the source line side is connected to an interconnect line SGS·M1extending in the X-direction through a contact plug P_(SGS), and theword lines WL<0>, WL<1>, WL<2>, and WL<3> are connected to interconnectlines WL<0>·M1, WL<1>·M1, WL<2>·M1, and WL<3>·M1 extending in theX-direction through contact plugs P_(WL<0>), P_(WL<1>), P_(WL<2>),P_(WL<3>), respectively.

Further, select gate lines SGD<0>, . . . , SGD<5> on the bit line sideare connected to interconnect lines SGD<0>·M1, . . . , SGD<5>·M1extending in the X-direction through contact plugs P_(SGD<0>), . . . ,P_(SGD<5>), respectively.

The bit lines BL<0>, . . . , BL<m> and the interconnect lines SGS·M1,WL<0>·M1, WL<1>·M1, WL<2>·M1, and WL<3>·M1, SGD<0>·M1, . . . , SGD<5>·M1are comprised of, for example, metal.

FIG. 2 shows a plan view of the BiCS-NAND flash memory of FIG. 1.

The columnar active layers AA are disposed in an array-state when viewedfrom the upper surface of the semiconductor substrate and comprise amemory cell array 15. Although the NAND cell unit is formed in each ofthe active layers AA, it will be described later in detail.

WL drivers 11-i and 11(i+1) are connected to the word lines WL<0>,WL<1>, WL<2>, and WL<3> through the interconnect lines WL<0>·M1,WL<1>·M1, WL<2>·M1, WL<3>·M1 and drive them in write, in read, and inerase.

SGS drivers 12-i and 12-(i+1) are connected to the select gate line SGSon the source line side through the interconnect line SGS·M1. A SGDdriver 13 is connected to the select gate lines SGD<0>, . . . , SGD<5>on the bit line side through the interconnect lines SGD<0>·M1, . . . ,SGD<5>·M1.

An SL driver 14 is connected to the source diffusion layer 24 throughthe source line SL·M1.

In this layout, the WL drivers 11-i and 11-(i+1) and the SGS drivers12-i and 12-(i+1) are disposed on one end side of the memory cell array15 in the X-direction, and the SGD driver 13 is disposed on the otherend side of the memory cell array 15 in the X-direction in considerationof an increase of the number of transistors constituting the drivers asa peripheral circuit.

FIG. 3 is a sectional view taken along line III-III of FIG. 2.

An N-type well region (N-well) 22 and a P-type well region (P-well) 23are formed in a P-type semiconductor substrate (P-sub) 21. The sourcediffusion layer 24 is an N-type diffusion layer and formed in the P-typewell region 23.

An N-channel FET (for example, N-channel MOSFET) 25 is formed in theP-type semiconductor substrate 21, and a P-channel FET (for example,P-channel MOSFET) 26 is formed in the N-type well region 22. Thesetransistors comprise the peripheral circuit (for example, the drivers)formed in a peripheral portion of a memory cell array.

The select gate line SGS on the source line side and the word linesWL<0>, WL<1>, WL<2>, and WL<3> are connected to the transistorsconstituting the drivers through the interconnect line in a first metallayer M1 and through an interconnect line in a second metal layer M2 onthe first metal layer M1.

To explain the word line WL<3> as an example, the word line WL<3> isconnected to the N-channel FET 25 constituting a word line driverthrough the interconnect line WL<3>·M1 in the first metal layer M1 andthrough an interconnect line WL<3>·M2 in the second metal layer M2 onthe first metal layer M1.

Here, gate electrodes of the N-channel FET 25 and the P-channel FET 26are formed simultaneously with, for example, the select gate line SGS onthe source line side.

That is, the gate electrodes of the N-channel FET 25 and the P-channelFET 26 have the same structure and the same thickness as those of theselect gate line SGS on the source line side.

FIG. 4 is a sectional view taken along line IV-IV of FIG. 2.

One ends (lowermost portions) of the active layers (active areas) AA areconnected to the source diffusion layer 24 passing through the selectgate line SGS on the source line side, the word lines WL<0>, WL<1>,WL<2>, and WL<3>, and the select gate lines SGD<0>, SGD<5> on the bitline side, and the other ends (uppermost portions) thereof are connectedto a bit line BL<0>.

The active layers AA are formed columnarly in the Z-direction (directionvertical to the front surface of the semiconductor substrate), and theNAND cell unit NAND is formed in each of the active layers AA.

FIG. 5 shows an example of a structure of the NAND cell unit NAND.

A memory cell MC has a MONOS structure.

The MONOS structure means a memory cell structure having a chargeaccumulation layer comprised of an insulation material such as nitride.The charge accumulation layer has a multilayer structure (charge traplayers), and ONO (oxide/nitride/oxide) is exemplified here.

A select gate transistor ST has the same structure as that of, forexample, the memory cell MC.

However, a gate insulation film of the select gate transistor ST mayhave a structure different from that of the memory cell MC, i.e., mayhave a structure that includes no charge accumulation layer (forexample, a single silicon oxide film).

FIG. 6 shows a bird's eye view of the NAND cell unit.

One of the features of the NAND cell unit having a three dimensionalstructure resides in that each of the select gate line SGS on the sourceline side, the word lines WL<0>, WL<1>, WL<2>, and WL<3>, and the selectgate lines SGD<0>, . . . , SGD<5> on the bit line side has a structurefor surrounding each of the side surfaces of the columnar active layersAA.

Accordingly, even if the active layers AA are made, for example, thinnerand a larger number of the active layers AA are formed on thesemiconductor substrate to increase a capacity, a force for driving thetransistors constituting the NAND cell unit can be sufficiently secured.

FIG. 7 shows an equivalent circuit of the memory cell array.

Since the BiCS-NAND flash memory has a three dimensional structure, theequivalent circuit is shown in a three dimension.

A larger number of memory cells constituting a NAND column can morecontribute to an increase of the capacity. However, as the number of thememory cells constituting the NAND column is more increased, there is apossibility that the characteristics of the memory cells are dispersedin a manufacturing process from the characteristics of a BiCS structure.

When the dispersion of the characteristics is taken into consideration,the NAND column is comprised of a smaller number of the memory cells(for example, four memory cells, eight memory cells, and the like).Further, the same structure may be stacked on a structure shown by theequivalent circuit of FIG. 7.

FIG. 8 is a view showing BiCS-NAND in comparison with two-dimensionalNAND.

In the NAND flash memory having the two-dimensional structure(two-dimensional NAND), one NAND cell unit in one block is connected toone bit line BL, whereas in the BiCS-NAND, NAND cell units in one blockare connected to one bit line BL.

Accordingly, as explained below, one of the cell units in the one blockconnected to the one bit line BL is selected by the select gate linesSGD<0>, . . . , SGD<5> on the bit line side in a write operation and aread operation.

(2) Basic Operations

Basic operations of the BiCS-NAND flash memory of FIGS. 1 to 8 will beexplained.

Since the basic write, read, and erase operations are the same as thoseof the NAND flash memory having the two-dimensional structure, mattersspecific to the BiCS-NAND flash memory will be explained here.

The concept of one block of the BiCS-NAND flash memory is different fromthat of the NAND flash memory having the two-dimensional structure.

Although the one NAND cell unit in the one block is connected to the onebit line BL in the NAND flash memory having the two-dimensionalstructure, the NAND cell units in the one block are connected to the onebit line BL in the BiCS-NAND flash memory.

For example, in the plan view of FIG. 2, six NAND cell units(corresponding to the number of the active layers AA in the figure) inthe block BK<i+1> are connected to the bit line BL<0>.

Accordingly, in the write operation and the read operation, one of thesix NAND cell units in the block BK<i+1> connected to the bit line BL<0>must be selected.

The selection is performed by select gate lines SGD<0>, . . . , SGD<5>on the bit line side. The select gate lines SGD<0>, . . . , SGD<5> onthe bit line side are individually connected to six NAND cell units inthe Y-direction in the block BK<i+1>.

The erase operation is performed collectively to, for example, all thememory cells in one block like the NAND flash memory having thetwo-dimensional structure.

The BiCS-NAND flash memory can be applied to both a binary memory, whichstores binary data in one memory cell, and a multi-level memory whichstores multi-level data having a ternary or more value in one memorycell.

3. Embodiments

Embodiment of the present invention will be explained.

(1) Block Layout

FIG. 9 shows a first example of a block layout of a BiCS memory.

The block layout corresponds to, for example, the BiCS-NAND flash memoryof FIG. 1 and has a feature in that a select gate line driver on a bitline side is disposed on one end of a memory cell array and a word linedriver and a select gate line driver on a source line side are disposedon the other end of the memory cell array.

A driver 33L, a level shifter 34L, and an address decoder 35L aredisposed on one end of the memory cell array 31 in the X-direction (on aleft side). The driver 33L is a driver for driving a select gate lineSGD on the bit line side and includes a transfer transistor.

A driver 33R, a level shifter 34R, and an address decoder 35R aredisposed on the other end of the memory cell array 31 in the X-direction(on a right side). The driver 33R is a driver for driving a word line WLand a select gate line SGS on the source line side and includes atransfer transistor.

Further, a sense amplifier 32 is disposed on one end of the memory cellarray 31 in the Y-direction. Bit lines BL<0>, . . . , BL<m> extending inthe Y-direction are disposed on the memory cell array 31 and connectedto the sense amplifier 32.

Since the block layout corresponds to the BiCS-NAND flash memory of FIG.1, the select gate lines SGD on the bit line side are disposed in blocksBK<0>, BL<1>, . . . , BK<n>, respectively. That is, since the area ofthe driver for driving the select gate lines SGD on the bit line side isincreased, a select gate line driver on the bit line side is disposed onthe one end of the memory cell array 31 in addition to the word linedriver and the select gate line driver on the source line side disposedon the other end of the memory cell array 31.

FIG. 10 shows a second example of the block layout of the BiCS memory.

A feature of the second example resides in that drivers 33 for driving aword line WL, a select gate line SGS on a source line side, and a selectgate line SGD on a bit line side are disposed together on one end of amemory cell array 31 in comparison with the first example.

The drivers 33, a level shifter 34, and an address decoder 35 aredisposed on one end of the memory cell array 31 in the X-direction (on aleft side). The drivers 33 are drivers for driving the word line WL, theselect gate line SGS on the source line side, and the select gate lineSDG on the bit line side and include transfer transistors.

Further, a sense amplifier 32 is disposed on one end of the memory cellarray 31 in the Y-direction. Bit lines BL<0>, . . . , BL<m> extending inthe Y-direction are disposed on the memory cell array 31 and connectedto the sense amplifier 32.

Since the drivers 33 for driving the word line WL, the select gate lineSGS on the source line side and the select gate line SGD on the bit lineside are disposed together as described above, an address decoder 34 anda level shifter 35 can be also disposed together at the same position.As a result, a layout of a peripheral circuit can be made efficiently.

However, in the above block layout, when the size of the memory cellarray 31 is increased and further a memory cell is miniaturized and theword line WL, the select gate line SGS on the source line side and theselect gate line SGD on the bit line side are disposed at narrowpitches, a problem arises in that a signal is delayed by a parasiticcapacitance.

In particular, in the BiCS memory, the word line WL and the select gateline SGS on the source line side are formed in a plate shape.Accordingly, coupling noise is caused by an increase of a parasiticcapacitance between the conductive wires.

FIG. 11 shows a third example of the block layout of the BiCS memory.

A feature of the third example resides in that two memory cell arrays31L and 31R are disposed in comparison with the second example. Sincedisposition of the two memory cell arrays 31L and 31R can reduce thelengths of a word line WL, a select gate line SGS on a source line side,and a select gate line SGD on a bit line side in each memory cell array,a signal delay and coupling noise can be suppressed.

The memory cell arrays 31L and 31R are disposed in the X-direction sideby side. Drivers 33L and 33R, a level shifter 34, and an address decoder35 are interposed between the memory cell arrays 31L and 31R. Thedrivers 33L and 33R drive the word line WL, the select gate line SGS onthe source line side, and the select gate line SGD on the bit line sideand include transfer transistors.

Further, sense amplifiers 32L and 32R are disposed on one ends of thememory cell arrays 31L and 31R in the Y-direction. Bit lines BL<0>, . .. , BL<m> extending in the Y-direction are disposed on the memory cellarrays 31L and 31R and connected to the sense amplifiers 32L and 32R.

Although the level shifter 34 and the address decoder 35 are shared bythe two memory cell arrays 31L and 31R in the block layout, the driver33L is disposed in correspondence with the memory cell array 31L, andthe driver 33R is disposed in correspondence with the memory cell array31R.

A reason why the drivers 33L and 33R cannot be shared by the two memorycell arrays 31L and 31R as described above is that since the drivers arecomprised of a lot of transistors, when they are shared by the memorycell arrays 31L and 31R, a layout of wirings for connecting the memorycell arrays 31L and 31R to the drivers 33L and 33R is made complex. Thatis, when a driver is disposed on each memory cell array, a layout ofwirings is more simplified than a case in which the drivers are sharedby the two memory cell arrays.

FIG. 12 shows a fourth example of the block layout of the BiCS memory.

A feature of the fourth example resides in that a driver 33 for drivinga word line WL, a select gate line SGS on a source line side, and aselect gate line SGD on a bit line side is shared by two memory cellarrays 31L and 31R in comparison with the third example.

The driver 33 can be shared by reducing the area thereof, i.e., byreducing the number of transistors constituting the driver 33. Morespecifically, this is because a layout of wirings for connecting thememory cell arrays 31L and 31R to the drivers 33L and 33R is not madecomplex by reducing the number of the transistors. Although the numberof the transistors constituting the driver 33 is reduced by a layout ofthe select gate line on the bit line side according to the presentinvention, this will be described later and only the block layout willbe explained here.

The memory cell arrays 31L and 31R are disposed in the X-direction sideby side. The driver 33, a level shifter 34, and an address decoder 35are interposed between the memory cell arrays 31L and 31R. The driver 33drives the word line WL, the select gate line SGS on the source lineside and the select gate line SGD on the bit line side and includes atransfer transistor.

Further, sense amplifiers 32L and 32R are disposed on one ends of thememory cell arrays 31L and 31R in the Y-direction. Bit lines BL<0>, . .. , BL<m> extending in the Y-direction are disposed on the memory cellarrays 31L and 31R and connected to the sense amplifiers 32L and 32R.

In the block layout, the driver 33, the level shifter 34, and theaddress decoder 35 are shared by the two memory cell arrays 31L and 31R.

Note that if the driver 33 can be shared by the two memory cell arrays31L and 31R without making a wiring layout complex, the block layout ofthe fourth example will be most preferable in the first to fourthexamples.

(2) Layout of Select Gate Line on Bit Line Side

FIG. 13 shows a first example of a layout of select gate lines on a bitline side. FIG. 14 shows a layout viewed on a plan view when the selectgate lines shown in FIG. 13 are arranged as a device.

The first example corresponds to the block layout of FIG. 9. That is, adriver 33L connected to select gate lines SGD<0>, . . . , SGD<5> on abit line side is disposed on one end (left side) of a memory cell array31 in the X-direction as well as disposed independently of a driver 33Rconnected to word lines WL<0>, . . . , WL<3> and to a select gate lineSGS on a source line side.

Each of two blocks BK<i>, BK<i+1> is comprised of at least threeconductive layers, which are insulated from each other and stacked on asemiconductor substrate, bit lines BL<0>, . . . , BL<m>, which areinsulated from the at least three conductive layers and disposedthereon, and active layers (columnar semiconductors) AA whose lower endsare connected to the semiconductor substrate, whose upper ends areconnected to the bit lines BL<0>, . . . , BL<m>, and which pass throughthe at least three conductive layers.

The uppermost layer of the at least three conductive layers is comprisedof the select gate lines SGD<0>, . . . , SGD<5> on the bit line side,the lowermost layer of the at least three conductive layers is theselect gate line SGS on the source line side, and the remainingconductive layers excluding the uppermost and lowermost layers of the atleast three conductive layers are the word lines WL<0>, . . . , WL<3>.

In the first example, although the number of the select gate linesSGD<0>, . . . , SGD<5> on the bit line side is six and the number of theword lines WL<0>, . . . , WL<3> is four in one block, respectively, thenumbers are not limited thereto. That is, it is sufficient that thenumber of the select gate lines on the bit line side and the number ofthe word lines be at least one in the one block, respectively.

Further, the remaining conductive layers excluding the uppermost layerof the at least three conductive layers have a plate shape whose widthin the Y-direction is larger than that in the Y-direction of the selectgate lines SGD<0>, . . . , SGD<5> on the bit line side.

Select gate transistors on the bit line side are comprised of the selectgate lines SGD<0>, . . . , SGD<5> on the bit line side and the activelayers AA, and select gate transistors on the source line side arecomprised of the select gate line SGS on the source line side and theactive layers AA. Further, memory cells are comprised of the word linesWL<0>, . . . , WL<3> and the active layers AA.

Further, the region between the memory cell array 31 and the driver 33Lis arranged as an interconnect portion 36L in which interconnect lines(conductive wires) SGD<0>·M1, . . . , SGD<5>·M1 are disposed to connectthe memory cell array 31 to the driver 33L. Likewise, the region betweenthe memory cell array 31 and the driver 33R is arranged as aninterconnect portion 36R in which interconnect lines (conductive wires)WL<0>·M1, . . . , WL<3>·M1, and SGS·M1 are disposed to connect thememory cell array 31 to the driver 33R.

The select gate lines SGD<0>, . . . , SGD<5> on the bit line side in theblock BK<i> and the select gate lines SGD<0>, . . . , SGD<5> on the bitline side in the block BK<i+1> are connected to the driver 33L afterthey are commonly connected in the relation of one to one in one end inthe X-direction of the memory cell array 31.

Specifically, an i-th (i is a natural number) select gate line on thebit line side from the block BK<i+1> side of the select gate linesSGD<0>, . . . , SGD<5> on the bit line side in the block BK<i> iscommonly connected to an i-th select gate line on the bit line side fromthe block BK<i> side of the select gate lines SGD<0>, . . . , SGD<5> onthe bit line side in the block BK<i+1>.

Accordingly, the select gate lines SGD<0>, . . . , SGD<5> on the bitline side have a folded layout in their entirety.

The folded layout can be easily formed by making use of, for example, aside wall masking technology for etching a ground layer using a sidewall as a mask.

FIG. 15 shows a second example of a layout of select gate lines on a bitline side. FIG. 16 shows a layout viewed on a plan view when the selectgate lines shown in FIG. 15 are arranged as a device.

The second example corresponds to the block layout of FIGS. 10 to 12.That is, a driver 33 (33L and 33R) disposed on one end (right side) of amemory cell array 31 in the X-direction is connected to word linesWL<0>, . . . , WL<3>, a select gate line SGS on a source line side, andselect gate lines SGD<0>, . . . , SGD<5> on a bit line side.

Attention must be paid to the fact that the portions, to which thelayouts of FIGS. 15 and 16 are applied as they are, are limited to theportion between the memory cell array 31L and the driver 33L of FIG. 11and to the portion between the memory cell array 31L and the driver 33of FIG. 12.

The layouts, which are obtained by reversing the layouts of FIGS. 15 and16 in a right to left direction, are applied to the remaining portionsbetween the memory cell array 31 and the driver 33 of FIG. 10, betweenthe memory cell array 31R and the driver 33R of FIG. 11, and between thememory cell array 31R and the driver 33 of FIG. 12.

Each of two blocks BK<i>, BK<i+1> is comprised of at least threeconductive layers, which are insulated from each other and stacked on asemiconductor substrate, bit lines BL<0>, . . . , BL<m>, which areinsulated from the at least three conductive layers and disposedthereon, and active layers (columnar semiconductors) AA whose lower endsare connected to the semiconductor substrate, whose upper ends areconnected to the bit lines BL<0>, . . . , BL<m>, and which pass throughthe at least three conductive layers.

The uppermost layer of the at least three conductive layers is comprisedof the select gate lines SGD<0>, . . . , SGD<5> on the bit line side,the lowermost layer of the at least three conductive layers is a selectgate line SGS on the source line side, and the remaining conductivelayers excluding the uppermost and lowermost layers of the at leastthree conductive layers are the word lines WL<0>, . . . , WL<3>.

In the second example, although the number of the select gate linesSGD<0>, . . . , SGD<5> on the bit line side is six and the number of theword lines WL<0>, . . . , WL<3> is four in one block, respectively, thenumbers are not limited thereto. That is, it is sufficient that thenumber of the select gate lines on the bit line side and the number ofthe word lines be at least one in the one block, respectively.

Further, the remaining conductive layers excluding the uppermost layerof the at least three conductive layers have a plate shape whose widthin the Y-direction is larger than the width in the Y-direction of theselect gate lines SGD<0>, . . . , SGD<5> on the bit line side.

Select gate transistors on the bit line side are comprised of the selectgate lines SGD<0>, . . . , SGD<5> on the bit line side and the activelayers AA, and select gate transistors on the source line side arecomprised of the select gate line SGS on the source line side and theactive layers AA. Further, memory cells are comprised of the word linesWL<0>, . . . , WL<3> and the active layers AA.

Further, the region between the memory cell array 31 (31L and 31R) andthe driver 33L (33L and 33R) is arranged as an interconnect portion 36in which interconnect lines (conductive wires) WL<0>·M1, . . . ,WL<3>·M1, SGS·M1, SGD<0>·M1, . . . , SGD<5>·M1 are disposed to connectthe memory cell array 31 to the driver 33.

The select gate lines SGD<0>, . . . , SGD<5> on the bit line side in theblock BK<i> and the select gate lines SGD<0>, . . . , SGD<5> on the bitline side in the block BK<i+1> are connected to the driver 33 (33L and33R) after they are commonly connected in the relation of one to one inone end in the X-direction (right side) of the memory cell array 31.

Specifically, an i-th (i is a natural number) select gate line on thebit line side from the block BK<i+1> side of the select gate linesSGD<0>, . . . , SGD<5> on the bit line side in the block BK<i> iscommonly connected to an i-th select gate line on the bit line side fromthe block BK<i> side of the select gate lines SGD<0>, . . . , SGD<5> onthe bit line side in the block BK<i+1>.

Accordingly, the select gate lines SGD<0>, . . . , SGD<5> on the bitline side have a folded layout in their entirety.

The folded layout can be easily formed by making use of, for example, aside wall masking technology for etching a ground layer using a sidewall as a mask.

FIG. 17 shows a layout in which drivers are disposed on both the sidesof a memory cell array.

As shown in FIG. 17, when the drivers 33 are disposed on both the sidesof the memory cell array 31, the size of the drivers 33 in theY-direction per, for example, one block can be increased (the number oftransistors can be increased). As a result, since the size of thedrivers 33 in the X-direction can be decreased (the number oftransistors can be decreased), the layout of interconnect lines(conductive wires) WL<0>·M1, . . . , WL<3>·M1, SGS·M1, SGD<0>·M1, . . ., SGD<5>·M1 in an interconnect portion 36 is further simplified.

Note that whether the drivers 33 (33L and 33R) are disposed on one sidesof the memory cell arrays 31 (31L and 31R) as shown in FIGS. 10 to 12 orthe drivers 33 are disposed on both the sides of the memory cell array31 as shown in FIG. 17 is determined in consideration of thespecification of the BiCS memory (chip), the area efficiency of theperipheral circuit, and the like.

(3) Example of Driver Circuit

An example of a driver circuit will be explained using a BiCS-NAND flashmemory as an example.

FIG. 18 shows the example of the driver circuit.

It is assumed that each of memory cell arrays is arranged such that fourword lines are disposed in one block (four layers), eight select gatelines (one layer) are disposed on a bit line side, and one select gateline (one layer) is disposed on a source line side.

A driver 33 is comprised of a transfer transistor (high voltagetransistor) to which a high voltage is applied. Each of row decoders 35is comprised of an AND circuit and decodes an address signal ADDRESS.Level shifters 34 are connected between the driver 33 and the rowdecoders 35.

BSTON, VRDEC, RDECANDn<0>, and RDECANDn<1> are control signals forturning on and off transfer transistors, and SGD<7:0>, CGi<3:0>,CG(i+1)<3:0>, SGSi, SGS(i+1), VRDEC2, and SGDS are transfer voltages.

(4) Read Disturb and Read Method

A. Read Disturb

First, read disturb specific to a BiCS memory will be explained using aBiCS-NAND flash memory as an example.

FIG. 19 shows three blocks.

It is assumed that a block BK<i> is a selected block and blocks BK<i−1>and BK<i+1> are non-selected blocks.

The selected block BK<i> has a selected NAND cell unit NAND-selectincluding a memory cell to be read. The NAND cell unit NAND-select islocated at the intersection point where a selected bit line BL-selectand a selected select gate line SGD-select on the bit line sideintersect with each other.

In a read operation, a read potential Vcgrv is applied to a selectedword line WL-select in the selected block BK<i>, and a transferpotential Vread higher than the read potential Vcgrv is applied tonon-selected word lines WL-unselect in the selected block BK<i>. Theread potential Vcgrv is a potential for turning on or off the memorycell depending on the state of a threshold value of the memory cell, andthe transfer potential Vread is a potential for turning on the memorycell without depending on the state of the threshold value of the memorycell.

Since the NAND cell units in the block BK<i> share the word lines, theread potential Vcgrv and the transfer potential Vread are applied alsoto the non-selected NAND cell units other than the selected NAND cellunit in the block BK<i>.

When the read operation is performed, the relation of the potentials inthe block BK<i> is as shown in FIG. 20.

The read potential Vcgrv is applied to the selected word line WL-select,and the transfer potential Vread is applied to the non-selected wordlines WL-unselect. Further, Vsgd (for example, about 4V) is applied to aselected select gate line SGD-select on the bit line side as a potentialfor turning on select gate transistors on the bit line side, and Vss(for example, 0V) is applied to non-selected select gate linesSGD-unselect on the bit line side as a potential for turning off theselect gate transistors on the bit line side. Further, Vsgs (forexample, about 4V) is applied to a select gate line SGS on the sourceline side as a potential for turning on select gate transistors on thesource line side.

Since the selected bit line BL-select is previously precharged with aprecharge potential Vpre, the potential of the selected bit lineBL-select is varied depending on the state of a threshold value of aselected memory cell MC-select. The data of the selected memory cellMC-select is determined by detecting the variation by a sense amplifier.

At this time, read disturb is liable to occur particularly in a memorycell X2 located adjacent to the bit line BL-select side of anon-selected memory cell X1 connected to the word line WL-select towhich the read potential Vcgrv is applied.

B. Conventional Read Method (Reference Example)

Occurrence of read disturb will be specifically explained.

FIG. 21 is a timing chart showing a conventional read method. FIGS. 22and 23 show the relation of potentials in read.

First, in a selected block BK<i>, a read potential Vcgrv is applied toselected word lines WL<2>, and a transfer potential Vread is applied tonon-selected word lines WL<0>, WL<1>, WL<3>. Further, a potential Vsgd(for example, 4V) is applied to select gate lines SGD on bit line sidesin selected NAND cell units NAND-select1 and NAND-select2 to turn onselect gate transistors on the bit line sides (solid line A), and apotential Vss (for example, 0V) is applied to select gate lines SGD onthe bit line sides in non-selected NAND cell units NAND-unselect to turnoff the select gate transistors on the bit line sides (broken line B).Further, the ground potential Vss is applied to source lines SL.

When the ABL (All Bit Line) sense system is employed, a prechargepotential Vpre is applied to all the bit lines BL<0> and BL<1> as shownin FIG. 22, and a potential Vsgs is applied to select gate lines SGS onthe source line sides to turn on select gate transistors on the sourceline sides (solid line C1 of FIG. 21). In contrast, when a sealed bitline sense system is employed, as shown in FIG. 23, the potential Vsgsis applied after the precharge potential Vpre is applied to selected bitlines BL<0>, the sealed potential Vss (for example, 0V) is applied tonon-selected bit lines BL<1>, the potential Vsgs is applied to selectedselect gate lines SGS on the source line sides to turn on the selectgate transistors on the source line sides, and the potential Vss isapplied to non-selected select gate lines SGS on the source line sidesto turn off the select gate transistors on the source line sides (brokenline C2 of FIG. 21).

As to non-selected blocks BK<i−1>, BK<i+1>, the relation of potentialsis as shown in FIGS. 22 and 23.

At this time, the select gate transistors on the bit line side areturned off in the non-selected NAND cell units NAND-unselect in theselected block BK<i> and cut off from the bit lines in read. Further,when a non-selected memory cell X1, to which the read potential Vcgrv isapplied, in the non-selected cell units NAND-unselect, is in a highthreshold value state, i.e., for example, a “0” state of a binary value(“0”/“1”) stored therein, the non-selected memory cell X1 is turned offbefore and after the read potential Vcgrv is applied thereto.

Accordingly, the channel of a memory cell X2 existing nearer to the bitline side than the memory cell X1, to which the read potential Vcgrv isapplied, in the non-selected cell unit NAND-unselect, is in a floatingstate as well as the channel thereof is boosted by capacitance couplingwhen the read potential Vcgrv is applied (Vboost).

In contrast, in a BiCS memory, the ground potential Vss is transferredfrom the source line SL to the channel of a memory cell existing nearerto the source line side than the memory cell X1, to which the readpotential Vcgrv is applied, in the non-selected NAND cell unitNAND-unselect in the read operation.

Accordingly, since a large potential difference is generated between asource and a drain of the memory cell X1, to which the read potentialVcgrv is applied, in the non-selected NAND cell unit NAND-unselect,punch-through is caused between the source and the drain of the memorycell X1.

A hot carrier is generated by the punch-through and injected into acharge accumulation layer of the memory cell X2 existing nearer to thebit lines BL<0>,BL<1> side than the memory cell X1, to which the readpotential Vcgrv is applied, in the non-selected NAND cell unitNAND-unselect, and a threshold voltage is varied thereby.

FIG. 24 shows a state when read disturb occurs.

FIG. 24 (a) shows a NAND cell unit including selected memory cellsMC-select1 and MC-select2, and FIG. 24 (b) shows a NAND cell unitincluding only non-selected memory cells. In FIG. 24 (b), memory cellsX1, X2 correspond to the memory cells X1, X2 of FIGS. 22 and 23.

C. Read Method according to Example of Present Invention

To prevent the read disturb described above, it is only necessary to setoperation timing at which generation of a large potential difference isprevented between the source and the drain of the memory cell, to whichthe read potential is applied, in the non-selected cell unit.

FIG. 25 is a timing chart showing a read method according to the exampleof the present invention.

FIGS. 26 and 27 show the relation of potentials in read.

Period 1

In a selected block BK<i>, a read potential Vcgrv is applied to aselected word line WL<2>, and a transfer potential Vread is applied tonon-selected word lines WL<0>, WL<1>, WL<3>. Further, a potential Vsgd(for example, 4V) is applied to all the select gate lines SGD on bitline sides to turn on select gate transistors on the bit line sides, apotential Vsgs (for example, 4V) is applied to all the select gate linesSGS on source line sides to turn on select gate transistors on thesource line sides, and a ground potential Vss is applied to all the bitlines BL<0>, BL<1>, and the source lines SL.

The read method according to the example of the present invention isdifferent from the conventional read method in that the read potentialVcgrv or the transfer potential Vread is applied to the word linesWL<0>, WL<1>, WL<2>, WL<3> in the selected block BK<i> in a state thatthe ground potential Vss is applied to a channel of a memory cell X2existing nearer to the bit lines BL<0>, BL<1> side than a memory cellX1, to which the read potential Vcgrv is applied, in the non-selectedNAND cell unit NAND-unselect in the selected block BK<i>.

In this case, when the read potential Vcgrv or the transfer potentialVread is applied to the word lines WL<0>, WL<1>, WL<2>, and WL<3> in theselected block BK<i>, the channel of the memory cell X2 in thenon-selected NAND cell unit NAND-unselect is fixed to the groundpotential Vss, and thus the channel of the memory cell X2 is notboosted.

Periods 2 to 3

The potential Vsgd (for example, 4V) is applied to the select gate linesSGD on the bit line sides in selected NAND cell units NAND-select1 andNAND-select2 to turn on the select gate transistors on the bit linesides (solid line A), and the potential Vss (for example, 0V) is appliedto the select gate lines SGD on the bit line sides in the non-selectedNAND cell units NAND-unselect to turn off the select gate transistors onthe bit line sides (broken line B). Further, the ground potential Vss isapplied to the source lines SL.

When the ABL (All Bit Line) sense system is employed, a prechargepotential Vpre is applied to all the bit lines BL<0> and BL<1> as shownin FIG. 26, and the potential Vsgs (for example, 4V) is applied to theselect gate lines SGS on the source line sides to turn on select gatetransistors on the source line sides (solid line C1 of FIG. 25). Incontrast, when the sealed bit line sense system is employed, thepotential Vsgs is applied after the precharge potential Vpre is appliedto the bit line BL<0> selected as shown in FIG. 27, the sealed potentialVss (for example, 0V) is applied to the non-selected bit line BL<1>, thepotential Vsgs is applied to the selected select gate lines SGS on thesource line sides to turn on the select gate transistors on the sourceline sides, and the potential Vss is applied to the non-selected selectgate lines SGS on the source line sides to turn off the select gatetransistors on the source line sides (broken line C2 of FIG. 25).

As to non-selected blocks BK<i−1>, BK<i+1>, the relation of potentialsis as shown in FIGS. 26 and 27.

At this time, the select gate transistors on the bit line sides areturned off in the non-selected NAND cell units NAND-unselect in theselected block BK<i> and cut off from the bit lines in read. Further,when a non-selected memory cell X1, to which the read potential Vcgrv isapplied, in the non-selected NAND cell units NAND-unselect, is in a highthreshold value state, i.e., for example, a “0” state of a binary value(“0”/“1”) stored therein, the non-selected memory cell X1 is turned off.

Accordingly, the channel of the memory cell X2 existing nearer to thebit line side than the memory cell X1, to which the read potential Vcgrvis applied, in the non-selected NAND cell unit NAND-unselect is in afloating state.

However, since the read potential Vcgrv and the transfer potential Vreadare already applied to the selected block BK<i> in the period 1, thechannel of the memory cell X2 is not boosted by capacitance coupling.

Accordingly, even if the ground potential Vss is transferred from thesource line SL to the channel of the memory cell existing nearer to thesource line side than the memory cell X1 in the non-selected NAND cellunit NAND-unselect, no large potential difference is generated betweenthe source and the drain of the memory cell X1. As a result, variationof a threshold voltage (read disturb) of the memory cell X2 isprevented.

FIG. 28 shows states when the read potential and the transfer potentialare applied.

FIG. 28 (a) shows a NAND cell unit including selected memory cellsMC-select1 and MC-select2, and FIG. 28 (b) shows a NAND cell unitincluding only non-selected memory cells. In FIG. 28 (b), memory cellsX1, X2 correspond to the memory cells X1, X2 of FIGS. 26 and 27, andnumerals in circles 1 to 3 correspond to the periods 1 to 3 of FIG. 25.

As apparent from FIG. 28 (b), when the read potential Vcgrv or thetransfer potential Vread is applied to the word lines WL<0>, WL<1>,WL<2>, and WL<3> in the selected block BK<i>, the channel of the memorycell X2 in the non-selected NAND cell unit NAND-unselect is fixed to theground potential Vss, and thus the channel of the memory cell X2 is notboosted.

D. Others

The read method according to the example of the present invention is notlimited to the operation timing of FIG. 25 and is effective in thefollowing cases.

In the period 1, the ground potential is supplied to the channel of thememory cell located nearer to the bit line side than the memory cell, towhich the read potential is applied, in the non-selected NAND cell unit.However, the ground potential may be supplied from the source line inplace of the above case. However, in this case, the transfer potentialVread is also applied to the selected word lines in the period 1.

It is sufficient that the read potential Vcgrv or the transfer potentialVread be applied to the word lines in the selected block before thetiming at which all the memory cells in the non-selected NAND cell unitin the selected block are cut off from the bit lines.

It is sufficient that the timing at which the precharge potential Vpreis applied to the bit lines be the same timing or after the timing atwhich all the memory cells in the non-selected NAND cell unit in theselected block are cut off from the bit lines.

(5) Conclusion

As described above, according to the embodiment of the presentinvention, read disturb of a three dimensional stacked nonvolatilesemiconductor memory to which a BiCS technology is applied can beimproved.

4. Application Example

Although the technology of the present invention is effective for aBiCS-NAND flash memory in which one cell unit is comprised of memorycells (NAND columns) connected to each other in series to realize bitcost scalability, the technology can be also applied to a threedimensional stacked nonvolatile semiconductor memory to which the BiCStechnology is applied in addition to the above BiCS-NAND flash memory.

For example, the technology of the present invention is also effectivefor a nonvolatile semiconductor memory which has exactly the same devicestructure as that of the BiCS-NAND flash memory but in which only onecentral memory cell of memory cells in one cell unit is used as a memorycell and the remaining memory cells are used as dummy cells as anexample other than the BiCS-NAND flash memory.

Further, as to a memory cell structure of the BiCS memory, it isconsidered that a so-called MONOS type, in which a charge accumulationlayer is comprised of an insulation material (for example, nitride), iseffective, but the example of the present invention is not limitedthereto and can be also applied to a floating gate type in which acharge accumulation layer is comprised of conductive polysilicon.

Further, a data value stored in one memory cell may be a binary value(two-level) or a multivalue (multi-level) of at least a ternary value(three-level).

5. Advantages

According to the present invention, read disturb of a three dimensionalstacked nonvolatile semiconductor memory to which a BiCS technology isapplied can be improved.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A nonvolatile semiconductor memory comprising: amemory cell array including first and second blocks; a first driver; thefirst block comprising a first cell unit including a selected memorycell and a first select gate transistor, and a second cell unitincluding an unselected memory cell and a second select gate transistor;a bit line to which the first and the second cell units are connected;and a word line electrically connected to a memory cell in the firstcell unit and connected to a memory cell in the second cell unit,wherein a read operation is performed with a first step and a secondstep, wherein, in the first step, a ground voltage is applied to the bitline, a first voltage is applied to the first select gate transistor,and a second voltage is applied to the second select gate transistor,the first and second voltages being higher than the ground voltage,wherein, in the second step, the ground voltage is applied to the secondselect gate transistor, the first voltage is applied to the first selectgate transistor, a third voltage is applied to the bit line, and a readvoltage or a transfer voltage higher than the read voltage is applied tothe word line in the first block, the third voltage being higher thanthe ground voltage.
 2. The memory according to claim 1, wherein thefirst cell unit and the second cell unit comprise a NAND cell unit. 3.The memory according to claim 1, further comprising: a source lineelectrically connected to the first cell unit and the second cell unit,wherein the ground voltage is applied to the source line in the firststep.
 4. A nonvolatile semiconductor memory comprising: a memory cellarray including first and second blocks; the first block comprising afirst cell unit including a selected memory cell and a first select gatetransistor, and a second cell unit including an unselected memory celland a second select gate transistor; a bit line to which the first andthe second cell units are connected; and a word line electricallyconnected to a memory cell in the first cell unit and connected to amemory cell in the second cell unit, wherein a read operation isperformed with a first step and a second step, wherein in the firststep, a read operation is performed by application of a read voltage ora transfer voltage higher than the read voltage to the word line in thefirst block, and a select gate connected to the second cell unit is putinto a conductive state, and wherein in the second step, the select gateconnected to the second cell unit is put into a cut off state and secondcell unit are cut off from the bit line, after which all the memorycells in the second cell unit are cut off from the bit line, the bitline is set to a precharge voltage, and read is performed to the memorycell to be read in the first cell unit, and wherein the read voltage andthe transfer voltage are not applied to the word line in the secondblock in the read.
 5. The memory according to claim 4, wherein theground voltage is transferred from the bit line to a channel of a memorycell existing nearer to a bit line side than a memory cell, to which theread voltage is applied, in the second cell unit.
 6. The memoryaccording to claim 4, wherein a timing at which the read voltage or thetransfer voltage is applied to the word line in the first block isbefore a timing at which all the memory cells in the second cell unitare cut off from the bit line.
 7. The memory according to claim 4,wherein a timing at which the precharge voltage is applied to the bitline is the same as or after a timing at which all the memory cells inthe second cell unit are cut off from the bit line.
 8. The memoryaccording to claim 4, wherein even after the read voltage or thetransfer voltage is applied to the word line in the first block, achannel of a memory cell existing nearer to a bit line side than amemory cell, to which the read voltage is applied, in the second cellunit is kept at the ground voltage.
 9. The memory according to claim 4,wherein the memory cell comprises a NAND cell unit.
 10. A nonvolatilesemiconductor memory comprising a memory cell array including first andsecond blocks; the first block comprising a first cell unit including aselected memory cell and a first select gate transistor, and a secondcell unit including an unselected memory cell and a second select gatetransistor; a bit line to which the first and the second cell units areconnected; and a word line electrically connected to a memory cell inthe first cell unit and connected to a memory cell in the second cellunit, wherein a read operation is performed with a first step and asecond step, wherein, in the first step, an initial voltage is appliedto the bit line, a first voltage is applied to the first select gatetransistor, and a second voltage is applied to the second select gatetransistor, the first and second voltages being higher than a groundvoltage, and a read voltage or a transfer voltage higher than the readvoltage is applied to the word line in the first block, wherein, in thesecond step, the ground voltage is applied to the second select gatetransistor, the first voltage is applied to the first select gatetransistor, a third voltage is applied to the bit line, the thirdvoltage being higher than the initial voltage.
 11. The memory accordingto claim 10, wherein the first cell unit and the second cell unitcomprise a NAND cell unit.
 12. The memory according to claim 10, furthercomprising: a source line electrically connected to the first cell unitand the second cell unit, wherein a voltage equal to the initial voltageapplied to the bit line in the first step is applied to the source linein the first step.